Setting collection metadata

Question

I'm trying to set metadata for a collection of the NFTS pallet. I'm doing this from a custom pallet.

To structure the metadata I'm using this guide: https://wiki.polkadot.network/docs/learn-nft-pallets

The guide stipulates that I should construct metadata as json with the following structure:

{
    "name":"Your Collection Name",
    "description":"Collection's Description",
    "image":"Your Collection CID"
}

How can I do this when using T::Nfts::set_collection_metadata(None, &T::ExperienceNFTCollectionId::get(), &metadata)?;?

I have tried many ways, but they all result in an IncorrectMetadata error. To be specific I tried:

• Creating a struct which I then SCALE encode. My believe is that this should be the right way, since it would be inefficient to store json bytes.
• Use a raw byte array of the json string (very inefficient)
• Use a SCALE encoded json string

Finally, I also replaced set_collection_metadata with setting each field individually with set_collection_attribute, but even that results in the aforementioned error for the image attribute.

T::Nfts::set_collection_attribute(&T::ExperienceNFTCollectionId::get(), b"name", b"Experience credentials")?;
T::Nfts::set_collection_attribute(&T::ExperienceNFTCollectionId::get(), b"description", b"Experience credentials")?;
T::Nfts::set_collection_attribute(&T::ExperienceNFTCollectionId::get(), b"image", b"https://cdn.pixabay.com/photo/2017/11/10/01/38/sand-2935107_960_720.jpg")?;

The guide uses IPFS, but I can hardly imagine that I'm obliged to use IPFS URLs for the image and that the problem if caused by using a random image URL for now. What am I doing wrong? What is the right way to set and encode collection metadata, as well as token metadata? I would prefer to use set_collection_metadata/set_item_metadata over setting each attribute individually unless there's no other choice.

Answer 1

It is failing with IncorrectMetadata because your provided metadata is too long, i.e longer than the StringLimit constant of pallet_nfts::Config. Metadata is stored as BoundedVec, which is bounded by StringLimit length. As far as I understood, you have the pallet_nfts configured in your runtime, so you can increase the StringLimit to some value that is enough for your needs.

impl pallet_nfts::Config for Runtime {
    ...
    type StringLimit = ConstU32<512>;
    ...
}